1. Field of the Invention
This invention relates to a piezoelectric body containing a ferroelectric substance phase having characteristics such that the ferroelectric substance phase is caused by electric field application to undergo phase transition. This invention also relates to a piezoelectric device provided with the piezoelectric body, and a liquid discharge apparatus provided with the piezoelectric device.
2. Description of the Related Art
Piezoelectric devices provided with a piezoelectric body, which has piezoelectric characteristics such that the piezoelectric body expands and contracts in accordance with an increase and a decrease in electric field applied across the piezoelectric body, and electrodes for applying the electric field across the piezoelectric body have heretofore been used as actuators to be loaded on ink jet type recording heads, and the like.
As piezoelectric body materials, there have heretofore been known perovskite type oxides, such as lead zirconate titanate (PZT). The materials described above are ferroelectric substances, which have spontaneous polarization characteristics at the time free from electric field application. With the conventional piezoelectric devices, ordinarily, an electric field is applied in a direction matched with a spontaneous polarization axis of the ferroelectric substance, and an ordinary electric field-induced strain extending in the direction of the spontaneous polarization axis is thereby utilized. Specifically, heretofore, it has been regarded to be important that material design be made such that the direction of the electric field application and the direction of the spontaneous polarization axis may coincide with each other (i.e., spontaneous polarization axis—direction of electric field application). However, in cases where the aforesaid piezoelectric effect of the ferroelectric substances is merely utilized, strain displacement of the piezoelectric device is limited. Therefore, nowadays there is a strong demand for piezoelectric devices having enhanced strain displacement.
Also, with size reduction and weight reduction made in electronic equipment and enhancement of functions made in electronic equipment in recent years, there has arisen a tendency toward the reduction in size and weight of piezoelectric devices and enhancement of functions of the piezoelectric devices. For example, in the cases of the piezoelectric devices for use in the ink jet type recording heads, such that images having good image quality may be obtained, it has recently been studied to enhance array density of the piezoelectric devices. Further, such that the array density of the piezoelectric devices may be enhanced, it has recently been studied to reduce the thicknesses of the piezoelectric devices. However, in the cases of the piezoelectric devices having the reduced thicknesses, if a voltage is applied across the piezoelectric device in the same manner as that for the conventional piezoelectric devices, the applied electric field exerted upon the piezoelectric body will become high. Therefore, in such cases, if the same material design as in the conventional piezoelectric devices is made directly, a sufficient piezoelectric effect will not be capable of being obtained.
FIG. 13 is a graph showing electric field-strain characteristics of conventional piezoelectric bodies. It has heretofore been known that the electric field-strain characteristics, which are obtained from the aforesaid ordinary electric field-induced strain of the ferroelectric substance, may be represented approximately by a curve IV as illustrated in FIG. 13. The curve IV indicates that, as for the applied electric field range of zero to a certain applied electric field Ex, the strain displacement increases linearly with respect to the increase in applied electric field. The curve IV also indicates that, as for the applied electric field range higher than the certain applied electric field Ex, the increase in strain displacement with respect to the increase in applied electric field becomes markedly small, and saturation is approximately reached in strain displacement.
Heretofore, the piezoelectric devices have been used with the applied electric field falling within the range of 0 to Ex, in which range the strain displacement increases linearly with respect to the increase in applied electric field. (For example, Ex takes a value of approximately 5 kV/cm to approximately 100 kV/cm, depending upon the kinds of the materials of the piezoelectric bodies, and the maximum applied electric field takes a value of approximately 0.1 kV/cm to approximately 10 kV/cm, depending upon the kinds of the materials of the piezoelectric bodies.) However, as for the piezoelectric devices having the reduced thicknesses, in cases where the voltage is applied across the piezoelectric device in the same manner as that for the conventional piezoelectric devices, the applied electric field exerted upon the piezoelectric body becomes high. Therefore, in such cases, the piezoelectric devices having the reduced thicknesses are used with the applied electric field falling within the range of, for example, 0 to Ey, where Ey>Ex. In such cases, a substantial piezoelectric constant may be represented by the inclination indicated by the broken line IV′ in FIG. 13. Specifically, in such cases, the substantial piezoelectric constant is smaller than the piezoelectric constant with respect to the applied electric field range of 0 to Ex, and the piezoelectric characteristics, which the piezoelectric devices originally have, are not capable of being utilized sufficiently.
In view of the above circumstances, a piezoelectric device, which utilizes a piezoelectric body having the characteristics such that the piezoelectric body is caused by electric field application to undergo phase transition, has been proposed in, for example, Japanese Patent No. 3568107. Japanese Patent No. 3568107 discloses a piezoelectric device comprising a phase transition film, electrodes, and a heating element for adjusting the temperature of the phase transition film at a temperature T in the vicinity of a Curie temperature Tc. (Reference may be made to Claim 1 of Japanese Patent No. 3568107.) Also, in Japanese Patent No. 3568107, as the phase transition film, there is mentioned a film, which undergoes the transition between a tetragonal system and a rhombohedral system, or the transition between the rhombohedral system or the tetragonal system and a cubic system. (Reference may be made to claim 2 of Japanese Patent No. 3568107.) In Japanese Patent No. 3568107, it is described that, with the piezoelectric device in accordance with the invention of Japanese Patent No. 3568107, strain displacement larger than with conventional piezoelectric devices is capable of being obtained by virtue of a piezoelectric effect of a ferroelectric substance and a volume change due to a change of a crystal structure accompanying the phase transition.
In Japanese Patent No. 3568107, as the phase transition film, there are mentioned the film undergoing the phase transition between the tetragonal system and the rhombohedral system, each of which constitutes the ferroelectric substance, and the film undergoing the phase transition between the rhombohedral system or the tetragonal system, which constitutes the ferroelectric substance, and the cubic system, which constitutes a paraelectric substance. However, the piezoelectric device described in Japanese Patent No. 3568107 is the one used at the temperature in the vicinity of the Curie temperature Tc. The Curie temperature Tc corresponds to the phase transition temperature between the ferroelectric substance and the paraelectric substance. Therefore, in cases where the piezoelectric device is used at the temperature in the vicinity of the Curie temperature Tc, the phase transition film will not be capable of undergoing the phase transition between tetragonal system and the rhombohedral system. Accordingly, the piezoelectric device described in Japanese Patent No. 3568107 will be the one utilizing the phase transition between the ferroelectric substance and the paraelectric substance. With the piezoelectric device utilizing the phase transition between the ferroelectric substance and the paraelectric substance, since the paraelectric substance does not have the spontaneous polarization characteristics, after the phase transition has occurred, the piezoelectric effect extending in the direction of the polarization axis will not be capable of being obtained from the electric field application.
The electric field-strain characteristics of the piezoelectric device described in Japanese Patent No. 3568107 may be approximately represented by a curve III as illustrated in FIG. 13. In FIG. 13, as an aid in facilitating the comparison, the electric field-strain characteristics before the phase transition occurs, which electric field-strain characteristics are represented by the curve III, are illustrated as being identical with the electric field-strain characteristics before the phase transition occurs, which electric field-strain characteristics are represented by the curve IV corresponding to the cases wherein only the ordinary electric field-induced strain of the ferroelectric substance is utilized. The curve III indicates that, as for the applied electric field range before the phase transition occurs, the strain displacement increases linearly with respect to the increase in applied electric field due to the piezoelectric effect of the ferroelectric substance. The curve III also indicates that, as for the applied electric field range of an applied electric field E4, at which the phase transition begins, to an applied electric field E5, at which the phase transition is approximately completed, the strain displacement increases due to the change of the crystal structure accompanying the phase transition. The curve III further indicates that, as for the applied electric field range higher than the applied electric field E5, at which the phase transition to the paraelectric substance is approximately completed, since the piezoelectric effect of the ferroelectric substance is not capable of being obtained anymore, the strain displacement does not increase with further application of the electric field. With the piezoelectric device described in Japanese Patent No. 3568107, as in the cases of the piezoelectric device utilizing only the ordinary electric field-induced strain of the ferroelectric substance, if the thickness of the piezoelectric device is reduced, the piezoelectric device having the reduced thickness will be used with the applied electric field containing the range of the high applied electric field, in which range little strain displacement is obtained, and the operation efficiency will not be capable of being kept high.
In Japanese Patent Application No. 2006-188765, the inventors have proposed a piezoelectric device, comprising:
a piezoelectric body having a polycrystalline structure containing first ferroelectric substance crystals, which have characteristics such that, with electric field application, at least a part of the first ferroelectric substance crystals undergo phase transition to second ferroelectric substance crystals of a crystal system different from the crystal system of the first ferroelectric substance crystals,
the piezoelectric device being actuated under conditions such that a minimum applied electric field Emin and a maximum applied electric field Emax satisfy Formula (1), preferably Formula (2):Emin<E4<Emax  (1)Emin<E4≦E5<Emax  (2)wherein the electric field E4 represents the minimum electric field at which the phase transition from the first ferroelectric substance crystals to the second ferroelectric substance crystals begins, and the electric field E5 represents the electric field at which the phase transition from the first ferroelectric substance crystals to the second ferroelectric substance crystals finishes approximately perfectly. (Reference may be made to Claim 1 or 2 of Japanese Patent Application No. 2006-188765.)
The electric field-strain characteristics of the piezoelectric device proposed in Japanese Patent Application No. 2006-188765 may be approximately represented by a curve II as illustrated in FIG. 13. As illustrated in FIG. 13, the piezoelectric body described in Japanese Patent Application No. 2006-188765 has the characteristics described below. Specifically, as for the range of the applied electric field E of 0 to E4 (i.e., at the stage before the phase transition occurs), the piezoelectric strain by virtue of the piezoelectric effect of the first ferroelectric substance crystals is obtained. Also, as for the range of the applied electric field E of E4 to E5, the piezoelectric strain by virtue of the phase transition is obtained. Further, as for the range of the applied electric field E of E≧E5 (i.e., at the stage after the phase transition has finished approximately perfectly), the piezoelectric strain by virtue of the piezoelectric effect of the second ferroelectric substance crystals is obtained. As described above, with the piezoelectric device described in Japanese Patent Application No. 2006-188765, the volume change due to the change of the crystal structure accompanying the phase transition of the piezoelectric body is obtained. Also, the piezoelectric body contains the ferroelectric substance crystals at both the stage before the phase transition occurs and the stage after the phase transition has occurred. Therefore, the piezoelectric effect of the ferroelectric substance is capable of being obtained at both the stage before the phase transition occurs and the stage after the phase transition has occurred. Accordingly, strain displacement larger than with the piezoelectric device described in Japanese Patent No. 3568107 is capable of being obtained.
In Japanese Patent Application No. 2006-188765, the inventors have indicated that the direction of the spontaneous polarization axis of the first ferroelectric substance crystals at the stage before the phase transition occurs should preferably be different from the direction of the electric field application. Also, the inventors have indicated that the direction of the electric field application should particularly preferably approximately coincide with the direction of the spontaneous polarization axis of the second ferroelectric substance crystals at the stage after the phase transition has occurred. (Reference may be made to Claims 3 and 4 of Japanese Patent Application No. 2006-188765.) In such cases, an “engineered domain effects” is capable of being obtained, and the phase transition proceeds efficiently. Therefore, enhanced strain displacement is capable of being obtained reliably.